The present invention relates to a method and a device for determining actual dimensional properties of a measured object having a plurality of geometric elements.
EP 2 738 515 A1 discloses a typical example of such a device in the form of a coordinate measuring machine. The known coordinate measuring machine has a workpiece receptacle and a CCD camera, which is movable in three orthogonal spatial directions relative to the workpiece receptacle. The CCD camera is part of a measuring head that can be used to determine the position of selected measurement points on a measured object relative to a reference coordinate system. Usually, such a device is used to determine measurement point coordinates for a plurality of measurement points on a measured object. On the basis of said measurement point coordinates, it is then possible to determine actual dimensional properties of the measured object, such as, for instance, the diameter of a bore or the distance between two edges on the measured object.
In principle, it is possible to manually control the movements of the measuring head relative to the measured object and the recording of the measurement values. However, an automated measurement sequence is desirable in the quality control of industrially produced products, such that a plurality of measured objects of identical type can be measured as rapidly and reproducibly as possible. Generating an automated measurement sequence requires fundamental knowledge of the operation of coordinate measuring machines and experience of how different geometric elements on a measured object can best be measured. Furthermore, an optimum measurement sequence may vary depending on what kind of measuring head and/or what movement axes are available on a coordinate measuring machine. In this regard, a measuring head having a non-contact sensor, as in the case of the coordinate measuring machine from EP 2 738 515 A1, for instance, may require a different measurement sequence than, for example, a tactile measuring head, i.e. a measuring head having a probe element configured to touch selected measurement points on the measured object.
EP 2 738 515 A1 proposes to use a wide-angle monitoring camera in addition to the CCD camera on the measuring head. The wide-angle monitoring camera is configured to record an image of the entire measured object from a bird's eye perspective. This (further) image is shown on the display of an operator terminal in order to make it easier for the operator to generate an automated measurement sequence taking into account and avoiding possible collisions between measuring head and measured object. Beyond that, the device from EP 2 738 515 A1 does not, however, offer more extensive support in the generation of a measurement sequence and the operator thus requires profound knowledge and experience in order to generate an optimum measurement sequence for a specific measured object.
Under the brand name CALYPSO, Carl Zeiss Industrielle Messtechnik GmbH offers software for generating an automatic measurement sequence and for processing the measurement results obtained. The basic principles of CALYPSO are described for example in a brochure entitled “Einfach Messen and was Sie dazu wissen sollten—Eine Fibel der Messtechnik” [“Straightforward measurement and what you should know to implement it—Primer for metrology”] (order number from Carl Zeiss: 61212-2400101) or in an advertisement brochure from Carl Zeiss Industrielle Messtechnik GmbH entitled “Calypso. Einfach programmieren” [“Calypso. Simple programming”] (publication number 60-11-068). The measurement sequence is generated by CALYPSO on the basis of so-called test features. A test feature represents an actual dimensional property of one or more geometric elements (so-called measurement elements) on a measured object, such as e.g. the diameter of a bore, the roundness of a cylinder section or the relative distance between two such geometric elements. As a rule, a plurality of measurement points on one or more geometric elements need to be captured in order to quantify a test feature. With the selection of a test feature, CALYPSO generates control commands that can be used to automatically control the measuring head for measuring the required measurement points. By means of the orientation to test features, CALYPSO makes it easier for an operator to generate the measurement sequence because the test features generally correspond to indications which the operator can infer from a technical drawing of the measured object.
For the purposes of generating the automated measurement sequence, the user must define and configure the desired test features one after the other using CAD data and, in this context, select various parameters including parameters for the machine movements. To this end, CALYPSO offers default values in each case, said default values having proven their worth for a large number of measurement problems, but an experienced user can modify the default values in order to optimize the automated measurement sequence in relation to a specific measurement problem.
Modifying a parameter in the course of configuring the measurement value recording for a first test feature may however affect the measurement accuracy and/or measurement speed for recording measurement values of another test feature since, for example, machine vibrations and/or approach paths may vary depending on the movement parameters for the first test feature. The generation of an ideal measurement sequence for a plurality of test features is therefore very time-consuming and requires significant specialist knowledge and experience on part of the user.
Completely independently of the problems and difficulties associated with generating a measurement sequence, there is a software tool, designated “Virtual CMM”, which was developed under the lead management of the Physikalisch-Technische Bundesanstalt (PTB), the national German metrology institute, for determining measurement uncertainty of a measurement with a coordinate measuring machine on the basis of statistical methods. By way of example, an overview of the Virtual CMM software tool is found, for example, in the publication “The ‘Virtual CMM’ a software tool for uncertainty evaluation—practical application in an accredited calibration lab” by Feinmess GmbH & Co. KG and the PTB.